The present invention relates to high-speed, high-resolution cinephotographic systems, and to cinephotographic systems for obtaining movies of combusting surface particle phenomena.
High speed movies are a useful tool in obtaining an understanding of the behavior of combusting surfaces, and especially in the study of solid propellant combustion surfaces. Solid propellants may generally be described as mixtures of particles of various types and sizes held together by a polymer binder. During combustion, these particles may interact with each other and with gases attending the combustion. In order to obtain a satisfactory understanding of the behavior of various propellant formulations for rocket motors, detailed photographic pictures of the combustion process of solid propellants are desirable; such pictures desirably utilize high resolution movies, including stereo or 3-D movies.
Existing methods for obtaining such high-speed movies are, however, severely limited in attainable resolution. Whereas typical solid propellant formulations may include particle sizes as small as two microns, existing movie technology can at best achieve 25 micron resolution--under special conditions, at low framing rates and in a limited field of view for a limited time. In addition to such resolution questions, it is significant to realize that previous systems for solid propellant photography have used flash lamp or xenon lamp illumination sources. Because these illumination sources have broad-band, white-light spectral properties one cannot segregate the flash illuminating light preferentially from over the broadband natural incandescence of the flame at the propellant surface. Solid propellant surface photography has also been additionally limited in resolution--because the flame from individual particles is so bright that one cannot see particles located below the flame--particles which are often smaller than the flame itself. Furthermore, in previous photography systems, the achieved shutter times are in the range of one microsecond or more in time duration. With such shutter times and with camera frames at a high rate, blur will occur in the propellant particle image that is attributable to motion of the film in the camera.
To overcome this motion blur problem, conventional cinema photography magnifies the image, and thereby achieves a greatly reduced field of view. However, since the depth of field in a photographic system decreases with the square of the employed magnification, these conventional photography systems become extremely limited in depth of field. Therefore, high-resolution propellant burn face movies using conventional cinephotography techniques have such a small depth of field that the burn face is in usable focus over only a very limited region of its surface and for a few frames in the course of an extended fixed focused movie sequence.
In contrast with these difficulties, the present system affords a 25 nonosecond effective shutter time and thereby affords motion freezing illumination. The present system is also of a high intensity and monochromatic nature. Additionally a photo-diode array servopositioner in the present system enables satisfactorily focused images to be obtained over a whole surface of the propellant sample and throughout the course of an entire movie sequence. The described system can therefore obtain 15-micron resolution at framing rates of over 7 kHz.
The patent art includes several examples of combustion and explosion study apparatus using both direct viewing and photographic techniques, and including both stereoptic and nonstereoptic imaging. Included in this patent art is the patent of Norman R. Zabel et al, U.S. Pat. No. 3,074,170, which concerns an arrangement for photographing explosion events through the use of high-speed photography, illumination by argon bomb light sources, and a rotating mirror framing arrangement.
The patent art also includes the patent of W. E. Buck et al, U.S. Pat. No. 3,366,439, which concerns a laser illumination and shuttering arrangement for high-speed photography. In the Buck et al patent, an argon gas pulsed laser is used in conjunction with a rotating mirror and a slitted stop plate for illuminating and photographing workpiece objects. The slitted stop plate in the Buck et al invention provides improved rise and fall times for the workpiece illumination. The Buck et al apparatus also contemplates photographic recording of a self-luminous event and employs monochromatic laser illumination with filters for suppressing the non-laser illumination.
The patent of G. H. McCall, U.S. Pat. No. 3,485,159, also discloses the use of a laser in photographing luminous gas surrounded objects. In the McCall apparatus a Kerr cell shutter and a Q-switched ruby laser are used to achieve exposure times in the 10 nanosecond range with frame separations as short as 5 nanoseconds.
The patent of Thomas W. Karras, U.S. Pat. No. 3,831,107 teaches the use of copper-vapor, quenched by cesium vapor in a laser.
The patent art has not, however, provided the combination of a high-speed well-focused motion picture system for rocket propellant samples, a system which employs a copper-vapor laser short pulse monochromatic illumination of the propellant sample burn face in a controlled atmosphere environment.